There's a leaflet included with your 8320HD that has a voucher code giving you a free month's subscription to Sky Player's Entertainment (Sky 1, Living and some others) and Sports (Sky Sports 1-4) Packs worth £34.50. Be warned that the offer closes on 31st December 2010 (though I wonder if new 8320HD boxes bought in 2011 will have a later expiry date?), so don't think you've got a whole year to sign up for your free month. Firstly, you have to register for a Sky ID - if you already subscribe to Sky, you'll have a Sky ID already. Theoretically, you could use someone else's Sky ID, but they'd be foolish to give it to you because you could sign up for all sorts - movies etc. - and cost them money.
A Sky ID allows up to four computers (Windows or Mac - they use Silverlight 4 which isn't available on Linux which only has Moonlight 3 so far) and a set-top box such as the 8320HD or an Xbox. To register the 8320HD as a play device, you simply go to channel 759 ("SKY") on your 8320HD, log in with your Sky ID and password and then navigate around. You can do the same with up to 4 PCs, but once you've registered them, they're strict about changing them (one change per month or something like that).
Once you have a Sky ID, there's an option in the accounts section to subscribe to various Sky packs. This is where you type in your promo code from your leaflet and it enables the aforementioned packs for free for one month. Despite it costing you nothing, you're forced - like FetchTV's registration - to put your credit card info in when you "buy" the packs for a month. Huge tip here - once you've signed up, go to the accounts section and immediately cancel the sub. They insist that you have to finish your current month and will not refund any part of it - they auto-cancel at the end of the month. Since you paid nothing, this still gives you the free month and Sky will then nicely cancel it without you doing anything more. If you don't do this cancellation, they will charge your credit card £34.50 at the start of the second month - you've been warned!
The Sky Player interface is reasonable apart from a couple of clunky issues. Firstly, the 8320HD takes an age (up to 2 seconds sometimes) to respond to any remote control key presses - it really does discourage any sort of surfing through channels. Secondly, all the live TV channels are numbered (e.g. MTV is 350) and yet the number buttons on the remote do absolutely nothing, which is frankly ridiculous. You have to cursor up and down on a Now/Next-style banner to find the channel you want (or go to the full Sky TV Guide)!
Other issues included a very slight motion blur and pixel artifacts if you sit close to your TV (I'm watching two live football matches today, so if they are affected by this, I'll comment on it and, yes, I'm always on the so-called 1.8Mbit/sec "high" quality that's actually slightly worse than SD!), being unable to get Sky Sports 1-4 in the early hours of this morning (screen went blank on those 4 channels, but were fine on all other channels I'm free-subbed to) and the inability to record anything. The Sky Player seems to overlay on the current Freeview channel you're watching, so pressing the record button will record the Freeview channel and not the Sky Player stream.
My conclusion? Reasonably OK for free, but there's absolutely no way I'd pay £34.50 a month for a barely-SD-quality unrecordable version of Sky Sports channels and a dozen or so rather poor "entertainment" channels. Up the picture quality, allow all programmes to be recordable (and not time-limited - forget that Sky Player desktop downloader thing) and throw in all the normal movie channels and it still would be too expensive!